The present invention refers to hubs for fans, wheels or the like, mainly consisting of a cast compound such as plastics, and a method for manufacturing such hubs.
Fans, wheels and similar devices are usually fitted to a steel shaft. The fitting on the shaft can be realized in different manners, but there is always a hub incorporating a bore through which the shaft is arranged. The hub may be fitted by means of screws, clamped by means of force fit or in other ways and it can be prevented from revolving by means of key in key groove. An indispensible condition at such a joint is that the connection shall not be weakened and break, even if the ambient temperature varies. Big temperature variations can occur particularly at fans depending upon the medium temporarily propelled by the fan. If hub and shaft are made from the same material, the temperature variations will not cause any essential problems, but if the two components are made from different materials having different thermal expansivity, then the grip of the hub about the shaft can vary.
Shafts are generally made from steel, whereas e.g. fans or their hubs are made from some sort of case or compression moulding compound. These materials, especially aluminum or plastic compound, have a much bigger thermal expansivity than steel. As to the plastics, these have, when subjected to load, a highly pronounced "creeping effect", which after a short time reduces the ability of the plastic to transfer forces and moments. It therefore has been necessary to take particular safety steps at fans with plastic hubs fitted to a steel shaft, in order to prevent these from coming loose, which can be fatal at high speeds. In order to give the hub sufficient strength it has also been necessary to produce it as a coarse lump. To make the hub as a coarse, heavily oversized lump however causes casting problems, higher manufacturing costs and provides no real solution on said problems of creeping effect and expansion due to temperature increase.